Aluminum methyl methylphosphonate (AMMP) is a flame retardant additive that has been used for various polymers.
Since AMMP does not melt during processing it is very important to achieve good dispersion of the AMMP in the polymer. It is well known in the art that good dispersion is achieved with fine powders with a narrow particle size distribution. Polymer composites with evenly distributed flame retardant particles have better flame retardant properties and better physical properties. If the particles are small (i.e., an average particle size of less than 5 microns) translucent or transparent polymer composites can be obtained.
Among the few known methods for preparing AMMP, the most frequently encountered ones are those based on the use of dimethyl methylphosphonate (DMMP) as a starting material. In one known method, DMMP reacts with an aqueous solution of a strong base, for example KOH or NaOH to form the water soluble alkali salt of methyl methylphosphonic acid. The same salts can also be prepared by reacting anhydrous DMMP with an alkali metal. The alkali salt of methyl methylphosphonic acid so obtained reacts further with a water soluble aluminum salt (e.g AlCl3 or Al2(SO4)3) to precipitate the water insoluble AMMP. This process requires water washing of the AMMP in order to remove sodium chloride or sulfate. The washing and filtering has to be repeated 4-5 times in order to achieve a high purity. This results in about 15 to 20% loss in the yield. Furthermore, dry AMMP forms a dense cake which must be milled in order to be useful as a flame retardant.
AMMP is also prepared by the reaction of DMMP with anhydrous aluminum chloride or an aluminum alkoxide, e.g. aluminum iso-propoxide. However, these preparations result in undesired and hazardous by-products such as methyl chloride, methylalkyl ethers, carbon monoxide, methane, and others. Furthermore, aluminum alkoxides are relatively expensive and difficult to handle.
Another approach for making AMMP involves the direct reaction of aluminum metal, in the form of aluminum foil with DMMP. This reaction requires prolonged heating (48 hours) at reflux, and the continuous addition of magnesium as a co-reactant. The resultant AMMP is contaminated with magnesium methyl methyl phosphonate.
Other processes can produce AMMP in a good yield by the reaction of DMMP with a finely divided form of aluminum hydroxide (ATH). According to these processes, the average particle size of the ATH has to be below 2 microns to make its reaction with DMMP more efficient, even though the reaction time is still undesirably long. Furthermore, such a reaction does not go to completion within a reasonable time even with ATH of such a small particle size. Unreacted ATH remains in the AMMP and therefore limits the application of the AMMP, because of the lower thermal stability of ATH. Such ATH consisting of such fine particles, is much more expensive compared to a variety of other coarser, but much cheaper grades of this product.
Another process comprises the reaction of DMMP with aluminum hydroxide having an average particle size of less than 2 microns, in the presence of large quantities of commercially unavailable methylphosphonic acid or methyl methylphosphonic acid. The latter is also used as a sole starting material for making AMMP by reaction with ATH in a polar organic solvent.